With Matt Harvey’s lower-back tightness no longer an issue, both he and Mets manager Terry Collins declared yesterday the ace right-hander is set to start Thursday against the Cardinals. “Absolutely,” Harvey said. Said Collins, “He’ll make his next start. I think that’ll be fine.” Harvey left Saturday’s game before the top of the eighth inning. “Everything’s fine,” he said yesterday. “Woke up and didn’t feel anything.” Harvey said when he was in college, the back issue occurred on occasion, but wouldn’t last. Still, Collins said he has to balance Harvey’s competitiveness and his safety. The manager said it’s one thing if Harvey’s pitching a critical game, but said, “Right now we can’t risk this guy getting hurt.” Jeremy Hefner will start tomorrow’s series opener against the Cardinals and Dillon Gee will pitch Wednesday. Shaun Marcum, who fired eight innings of relief in Saturday’s 20-inning loss, will start Friday. Collins said Harvey will start Thursday and Marcum on Friday because Marcum logged more pitches than Harvey in Saturday’s game (105 to 93). * Pitching for the first time since May 27 due to left shoulder tendinitis, Jonathon Niese allowed three runs (two earned) in 6 2/3 innings yesterday and said his shoulder was a non-issue during the game. “Felt really good,” said Niese, who will continue to receive treatment. “It felt strong.”